1. Field of the Invention
The present invention relates to a semiconductor light emitting device having a structure in which an n-type cladding layer, an active layer and a p-type cladding layer, respectively made of a Group II-VI compound semiconductor, are sequentially stacked and to a method of manufacturing same.
2. Description of the Related Art
In recent years, a demand for the high density, high resolution of recording and reproduction of an optical disc and a magneto-optic disc rises. There is also a marked tendency to develop a high brightness display device and a low-loss optical fiber communication device as well as an optical analytic device for DNA and a particular chemical substance. In response to them, the development of semiconductor light emitting devices capable of emitting green or blue light has been demanded as their light sources.
As materials for composing the green or blue light emitting semiconductor devices, Group II-VI compound semiconductors each of which comprises at least one element of Group II elements of zinc (Zn), magnesium (Mg), beryllium (Be), cadmium (Cd), mercury (Hg) and manganese (Mn) and of at least one of elements of Group VI of oxygen (O), sulfur (S), selenium (Se) and tellurium (Te) hold great promise.
To obtain a semiconductor light emitting device having high luminous efficiency, it is necessary to improve the crystallinity of the Group II-VI compound semiconductors. Among the Group II-VI compound semiconductors, mixed crystal of ZnMgSSe, in particular, can lattice-match with a substrate comprising GaAs or ZnSe which is excellent in crystallinity and easy to obtain, which enables it to improve crystallinity. The ZnMgSSe mixed crystal is, therefore, particularly known as a material for the guiding layer and the cladding layer of a semiconductor light emitting device (as shown in, for example, Electronic Letters 28 (1992), page 1,798).
The Group II-VI compound semiconductors are, however, generally difficult to have high carrier concentrations even if p-type impurities are added thereto. The mixed crystal of ZnMgSSe, for example, has a low carrier concentration of only about 1.times.10.sup.17 to 2.times.10.sup.17 cm.sup.-3. Among materials which lattice-match with GaAs, ZnSSe mixed crystal can have a higher carrier concentration than ZnMgSSe, however, at most, it is only about 7.times.10.sup.17 cm.sup.-3. For that reason, even if a p-side electrode is formed through a ZnSSe layer above a cladding layer made of ZnMgSSe mixed crystal, it is difficult to maintain an ohmic contact with the p-side electrode, thereby causing a rise in operating voltage. Due to this, power consumption increases and, at the same time, heat is generated disadvantageously resulting in a deterioration in device.
In the prior art in consideration of these disadvantages, Group II-VI compound semiconductor layers capable of obtaining high carrier concentrations without lattice-matching with GaAs are formed above the ZnSSe layer to lower operating voltage. A ZnSe layer, for example, capable of obtaining a carrier concentration of about 1.times.10.sup.18 cm.sup.-3 is formed on the ZnSSe layer and a ZnTe layer capable of obtaining a carrier concentration of about 1.times.10.sup.19 cm.sup.-3 is formed on the ZnSe layer.
The conventional semiconductor light emitting device can maintain an ohmic contact between the ZnTe layer and the p-side electrode. The carrier concentration of the ZnSe layer is, however, low and it is impossible to sufficiently lower operating voltage. As a result, the conventional device is disadvantageous in that sufficiently longer life time cannot be expected. The reason is considered to be the influence of the ZnTe layer.